Their current BBC3 sketch show has been drubbed by the critics - and there's no improvement on that programme's lobotomised humour at this live outing, a headline performance in aid of the Teenage Cancer Trust. The only person to emerge with reputation intact is Ruth Jones, Corden's co-writer and love interest on Gavin and Stacey. Tonight's set features only five sketches, interspersed between the other standups on the bill, Sean Lock and Noel Fielding. These skits include the Young People's Church, a happy-clappy Christian send-up whose raison d'etre is to mix religiosity with infantile double entendre, "Jesus, come in my back door", and so on (Horne and Corden are both 30 years old).

Also present is Horne's camp TV news incarnation, reporter Tim Goodall, a mincing gay stereotype who says things like: "Look at the size of Albert Hall. It's massive. Can you imagine the size of his cock?" This routine, in which Tim insults the dress sense of some members of the audience, culminates in a guest appearance by Gok Wan, over whom Corden fawns to no discernible comic purpose.

As my will to live fizzled away, I wondered: how has it come to this? Corden is a likable actor on stage and screen, but he's a loutish comic, all aggressive neediness and blokey bonhomie (one of their few slots tonight is given over to orchestrating a Mexican wave). Horne was a skilled deadpan foil to Catherine Tate as well as in Gavin and Stacey. But here, he looks dead behind those piercing eyes, as if his soul has decided to stay at home.

There's no spark, no dynamic relationship between the two to generate tension or comedy. Nor is there sensitivity, warmth – or the sense of one's own ridiculousness from which comedy springs. Their final sketch, in which two frilly magicians flounce around, performing crap tricks to a bombastic soundtrack, suggests they can't even make basic silliness funny. "Everybody is going down on you," sing their Young People's Church alter egos, with forced innuendo. But it's Horne and Corden who are going down – and fast. Surely they can't sink further.